The present invention relates to apparatus and method for encoding a digital video signal and, more particularly, to apparatus and method which motion compensates encodes a video signal pre-filtered by a first filter using motion information derived from the same video signal pre-filtered by a second filter.
As is known, digital video signals generally are compressed prior to being transmitted or recorded on a record medium. However, compression-encoding of a video signal causes a deterioration, for example, a block distortion or "ringing" of contour portions, of the video picture represented by the video signal. To reduce the loss of picture quality caused by such compression-encoding, it is common to supply the video signal through a low pass filter (called a pre-filter) prior to the compression thereof.
FIG. 1 is a block diagram of an encoder in which a digital video signal supplied to an input terminal 1 is applied to a pre-filter 2 before it is encoded in an encoding circuit 3. Pre-filter 2, which may be a low pass filter, removes various "noise" from the video signal and supplies the resultant filtered video signal (S11) to both hybrid encoding unit 31 and motion vector estimation (ME) unit 32. ME unit 32 detects motion in the video signal and supplies the detected motion as a motion vector S13 to hybrid encoding unit 31. Encoder 31, which generally is a motion compensating and transforming encoder, encodes the filtered video signal supplied from pre-filter 2 using motion vector S13 to produce an encoded video signal S12. The encoded video signal is supplied as an output at output terminal 4.
As is known, video signals are pre-filtered to prevent deterioration of the signal during the encoding thereof. Pre-filtering of the video signal also prevents noise that has been removed therefrom from otherwise erroneously being detected to represent motion in the video picture by ME unit 32.
One difficulty encountered in typical video signal encoders which include a pre-filter therein is their general inability to prevent a so-called "blurring" of video signals that have a high transmission bit rate, e.g., MPEG2 video signals, when the video signals are pre-filtered and subsequently encoded. Such blurring results in a loss of fine picture patterns which, in addition to lowering the picture quality, lowers the precision of motion detection by a motion detector, e.g., ME unit 32. On the other hand, if a video signal having a high transmission bit rate is not filtered before it is encoded, high frequency noise in the video signal erroneously is detected as motion therein.
If a video signal having a low transmission bit rate, e.g., an MPEG1 video signal, is filtered and encoded, the motion detection precision of ME unit 32 is lowered. On the other hand, if a video signal transmitted at a low bit rate is not pre-filtered, the amount of data in (i.e., the bit rate of) the resultant encoded video signal may exceed a desired bit rate or the bit rate at which the encoder is capable of transmitting.
Another difficulty encountered in the above-discussed video signal encoders is their general inability to encode video signals that are derived from a film source without substantially deteriorating those video signals. As is known, a picture signal derived from a film source has so-called grain noise (or granular noise) which adds "texture" to the film picture. Grain noise is desirable in video signals derived from a film source and is distinguished from other types of noise, for example, high frequency noise, which desirably should be removed from the video signal. When a motion video signal derived from a film source having either a high or a low transmission bit rate is filtered in a pre-filter prior to encoding, the pre-filter removes the high frequency noise, as well as the grain noise from the video signal. If a video signal having a low transmission bit rate is derived from a film source, pre-filtering thereof also undesirably lowers the motion detection precision of the motion detecting circuit, e.g., ME unit 32. However, if a video signal derived from a film source is not pre-filtered prior to being encoded, both high frequency noise and grain noise therein erroneously are detected to represent motion in the video signal. Further, the encoding of an unfiltered video signal transmitted at a low transmission bit rate may result in an encoded video signal with a transmission bit rate that exceeds that of the encoder or the accompanying circuits.
A further difficulty encountered in typical encoders is their general inability to encode motion video signals that represent multiple "scenes" that have been edited and combined therein without substantially deteriorating the video signals. Motion video that represents, for example, a television program, usually includes therein multiple scenes that have been produced by multiple cameras. The various scenes may represent, for example, a still picture, successive images having very high motion therein (e.g., an automobile chase), a fine textured image, a coarse textured image, etc. When a video signal that represents such different types of "scenes" is pre-filtered prior to being encoded, the quality of the resultant encoded picture is substantially lowered.